One of the hallmarks of contemporary civilization is that each increment of technological progress almost invariably is accompanied by a similar increment of environmental regress. As the pace of technological advances quickens so does the march of environmental deterioration. The realization of environmental damage has occurred only relatively recently, so that present society sometimes finds itself burdened with the accumulated sins of the not-too-distant past. But another hallmark of current society is its acceptance of the undesirability of environmental degradation coupled with a determination to minimize and even reverse it wherever possible. Although the return of ground waters to their pristine condition of an earlier era is not a realistic goal, there is a genuine determination to make our waters as pure as possible. Environmental agencies have set limits for many common industrial pollutants, and as methods of pollution reduction have become more successful in reducing or removing pollutants from waste water, environmental regulations have become more stringent, resulting in an ever tightening spiral whose goal is to reduce pollutants in waste water to that minimum which is technologically feasible.
Among the methods employed to reduce or remove pollutants, bioremediation constitutes an effective and highly desirable approach. Quite broadly in bioremediation pollutants serve as a food source, generally as a source of carbon and/or nitrogen, for microorganisms. Bacterial metabolism converts the pollutants to metabolites generally with a simple chemical structure, sometimes degrading the pollutants completely to carbon dioxide and water in an aerobic process, or to methane in an anaerobic process. But in any event the metabolites usually have no adverse environmental effects.
Phenol itself, and phenolic compounds as a generic class, is a commonly employed industrial chemical used by many industries, such as resin manufacturers. Phenols also are a product of delignification and are formed in large quantities in wood pulping processes. This application has as its particular focus removal of phenolic compounds, such as phenol, cresols, catechols, resorcinol, nitrophenols, and halogenated phenols, from waste water. However, discussion within will emphasize the suitability of the process which is our invention to a broad class of organic materials, including aromatic hydrocarbons and polynuclear aromatic hydrocarbons.
Bioremediation has been successfully and extensively used to reduce phenol levels, most commonly effecting phenol biodegradation using activated sludge in storage retention ponds. Phenols at high concentration are toxic to microorganisms, and addition of adsorbents such as Fuller's earth and activated carbon to a retention pond appears to prevent phenols at toxic concentrations from interfering with bacterial activities, or at least to reduce such interference; see U.S. Pat. Nos. 3,904,518 and 4,069,148. Despite its wide commercial acceptance this method has various disadvantageous consequences whose elimination would be welcome. Degradation of phenolics occurs slowly, with degradation time depending, inter alia, on the phenol concentration in the initial waste stream, but a holding time of 10-20 days is not unusual. In turn this requires very large storage basins, which means large land masses are dedicated to phenol degradation. Especially in urban areas, removal of scarce land masses from industrial development and other uses is difficult if not impossible. Phenol degradation by activated sludge also is attended by continued replication of the microorganisms leading to further sludge formation which needs to be continually removed from the retention basins and sludge disposal itself is a significant cost factor in phenolics removal from waste waters. Additionally, the activated carbon needs to be periodically replaced and/or regenerated, adding to the overall process cost both via its replacement as well as the disposal of spent carbon.
The aforementioned disadvantages have led others to seek bioremediation methods employing a higher biomass concentration than found in retention ponds. In particular, beds containing microorganisms "attached" to the support have been used in both aerobic and anaerobic degradation of phenol, with and without the presence of activated carbon. Despite the limited success which has been experienced, none of the prior art methods have matured into a commercially successful process. In contrast, we have found a means of using a porous biomass support system which overcomes many limitations of the prior art and which has great commercial potential.
The invention described within is applicable to the biodegradation of large classes of organic pollutants, nonetheless an important focus of our efforts is the removal of phenol and other phenolic materials from waste water. For the purpose of clarity in exposition and conciseness what follows usually will be couched in terms of phenol removal. However, at all times it is to be understood that phenol is but one phenolic compound, and phenolic materials are but representative of organic polluting components which can be removed by our invention. Consequently the invention is to be given far broader scope than merely the removal of phenolic materials.
As befits the current emphasis on environmental cleanup, a vast amount of research has been directed toward the removal of phenol. The following review is not intended to be exhaustive but rather discusses some representative pertinent art. Both U.S. Pat. Nos. 3,904,518 and 4,069,148 describe the addition of activated carbon or Fuller's earth to a suspension of biologically active solids (activated sludge) in waste water as an aid in phenol removal. The adsorbents presumably act by preventing pollutants toxic to the bacteria from interfering with bacterial metabolic activity. The patentees' approach has matured into the so-called PACT process which has gained commercial acceptance despite its requisites of a long residence time, copious sludge formation with attendant sludge disposal problems, and the need to regenerate and replace spent carbon.
Rehm and coworkers have further refined the use of activated carbon in the aerobic oxidation of phenolic materials by using microorganisms immobilized on granular carbon as a porous biomass support system. Utilizing the propensity of microorganisms to grow on and remain attached to a surface, Rehm used a granular activated carbon support of high surface area (1300 m.sup.2 /g) to which cells attached within its macropores and on its surface, as a porous biomass support system in a loop reactor for phenol removal. H. M. Ehrhardt and H. J. Rehm, Appl. Microbiol. Biotechnol., 21 32-6 (1985). The resulting "immobilized" cells exhibited phenol tolerance up to a level in the feed of about 15 grams per liter, whereas free cells showed a tolerance not more than 1.5 g/L. It was postulated that the activated carbon operated like a "buffer and depot" in protecting the immobilized microorganisms by absorbing toxic phenol concentrations and setting low quantities of the absorbed phenol free for gradual biodegradation. This work was somewhat refined using a mixed culture immobilized on activated carbon (A. Morsen and H. J. Rehm, Appl. Microbiol. Biotechnol., 26, 283-8 (1987)) where the investigators noted that a considerable amount of microorganisms had "grown out" into the aqueous medium, i.e., there was substantial sludge formation in their system.
Suidan and coworkers have done considerable research on the analogous anaerobic degradation of phenol using a packed bed of microorganisms attached to granular carbon (Y. T. Wang, M. T. Suidan and B. E. Rittman, Journal Water Pollut. Control Fed., 58, 227-33 (1986)). For example, using granular activated carbon of 16.times.20 mesh as a support medium for microorganisms in an expanded bed configuration, and with feed containing from 358-1432 mg phenol/L, effluent phenol levels of about 0.06 mg/L (60 ppb) were obtained at a hydraulic residence time (HRT) of about 24 hours. Somewhat later, a berlsaddle-packed bed and expanded bed granular activated carbon anaerobic reactor in series were used to show a high conversion of COD to methane, virtually all of which occurred in the expanded bed reactor; P. Fox, M. T. Suidan, and J. T. Pfeffer, ibid., 60, 86-92 (1988). The refractory nature of ortho- and meta-cresols toward degradation also was noted.
Givens and Sack, 42nd Purdue University Industrial Waste Conference Proceedings, pp. 93-102 (1987), performed an extensive evaluation of a carbon impregnated polyurethane foam as a microbial support system for the aerobic removal of pollutants, including phenol. Porous polyurethane foam internally impregnated with activated carbon and having microorganisms attached externally was used in an activated sludge reactor, analogous to the Captor and Linpor processes which differ only in the absence of foam-entrapped carbon. The process was attended by substantial sludge formation and without any beneficial effect of carbon.
The Captor process itself utilizes porous polyurethane foam pads to provide a large external surface for microbial growth in an aeration tank for biological waste water treatment. The work described above is the Captor process modified by the presence of carbon entrapped within the foam. A two-year pilot plant evaluation of the Captor process itself showed substantial sludge formation with significantly lower microbial density than had been claimed. J. A. Heidman, R. C. Brenner, and H. J. Shah, J. of Environmental Engineering, 114, 1077-96 (1988). A point to be noted, as will be revisited below, is that the Captor process is essentially an aerated sludge reactor where the pads are retained in an aeration tank by screens in the effluent line. Excess sludge needs to be continually removed by removing a portion of the pads via a conveyor and passing the pads through pressure rollers to squeeze out the solids.
H. Bettmann and H. J. Rehm, Appl. Microbiol. Biotechnol., 22, 389-393 (1985) have employed a fluidized bed bioreactor for the successful continuous aerobic degradation of phenol at a hydraulic residence time of about 15 hours using Pseudomonas putida entrapped in a polyacrylamide-hydrazide gel. The use of microorganisms entrapped within polyurethane foams in aerobic oxidation of phenol in shake flasks also has been reported; A. M. Anselmo et al., Biotechnology B. L., 7, 889-894 (1985). The latter appears to be the sole report of microorganisms entrapped within a foam used for biodegradation of organic pollutants.
An industrially desirable method of removing phenolic materials from waste waters would have the following characteristics. The method would be (1) an aerobic oxidation achieving (2) effluent phenol levels less than 0.1 parts per million (ppm) at (3) hydraulic residence times under 24 hours requiring (4) no activated carbon regeneration or replacement and with (5) substantially less sludge formation than obtained from currently available technology. None of the aforementioned art achieves all of the above, nor does the art give any indication how such a goal can be achieved. We have found that if both powdered activated carbon and phenol-degrading aerobic microorganisms are entrapped within an open-celled polyurethane foam which is then used as a porous biomass support system in a fixed bed reactor, each of the foregoing goals are readily attained. Levels of effluent phenol down to at least 20 parts per billion can be attained at an HRT of under about 16 hours. Carbon is not physically lost from the reactor, thus avoiding the need for replacement, and is self-regenerative within the reactor. Sludge formation is minimal; comparative tests with other fixed bed reactors show that our immobilized cell bioreactor (ICB) produces less than 25 percent the amount of sludge formed by presently commercially viable systems. In short, as measured by its performance characteristics our invention is a marked improvement over the prior art; relative to the prior art our invention represents a difference in kind rather than a difference in degree.
Reduced sludge formation attending our process is neither an incidental nor a minor benefit. A major result of increased wastewater treatment is an ever increasing quantity of sludge, which presents a serious disposal problem because of increasingly restrictive policies on dumping or spreading untreated sludge on land and at sea. The cost of sludge disposal today may be severalfold greater than the sum of other operating costs of wastewater treatment. Accordingly, the reduction in sludge levels characteristic of our invention has immediate, substantial economic benefit and alleviates the pressures of sludge dumping.